A base station in a mobile telephony system receives and transmits wideband high-frequency radio signals having a bandwidth of up to 30 MHz. The received wideband signal is separated into narrowband (for example 30 kHz wide) channels (FDMA) or channel groups (TDMA). In the reverse process channels or channel groups are combined into a wideband signal for transmission.
It has been suggested to perform A/D conversion of the entire wideband spectrum and to perform the channel separation digitally. For each channel or channel group the digital wideband signal is demodulated with a different frequency in order to shift this particular channel or channel group down to the baseband. The demodulation is performed with a quadrature network for generating the I and Q components. Thereafter these components are low-pass filtered in order to separate the desired channel or channel group from the unwanted neighbor channels or channel groups. Finally the samples of the separated signals are decimated (down-sampled).
In the reverse process baseband signals are interpolated (up-sampled), modulated and combined into a wideband signal.
A drawback of these methods is that the demodulation and modulation has to be performed at the high sampling frequency of the digital wideband signal, which requires a lot of data processing. Furthermore, the required local oscillators and multipliers require a lot of space and are power consuming.